The invention relates to improvements in methods of and in apparatus for laminating plastic foils. The invention also relates to laminates which are obtained in accordance with the improved method and by resorting to the improved apparatus. More particularly, the invention relates to improvements in methods of and in apparatus for making laminates from stretched and unstretched plastic foils.
It is already known to make laminates wherein a longitudinally (monoaxially) stretched first foil is bonded to an unstretched second foil. The thus obtained laminate exhibits highly satisfactory strength characteristics which are attributable to the presence of the stretched foil and are effective in the direction of stretch, as well as highly satisfactory strength characteristics in a direction transversely of the laminate. Moreover, the laminate exhibits a desirable pronounced rigidity. In accordance with heretofore known proposals, a laminate which consists of or includes a stretched and an unstretched foil is obtained by resorting to an adhesive as a means for bonding the foils to each other. The temperature at which the bonding step takes place must be below the crystallite melting point of the stretched foil in order to ensure that the beneficial results of stretching at a temperature below such point are not lost in the course of the bonding operation with an adhesive.
A drawback of the just outlined conventional methods is that the adhesive contributes significantly to the cost of making the laminate. In addition, recycling of the laminate presents problems due to the presence of adhesive therein.
Attempts to overcome the drawbacks of a laminate wherein a stretched foil is bonded to an unstretched foil by means of an adhesive include extrusion of a second foil directly against one side of an unstretched foil. However, the resulting laminate does not exhibit the characteristics of a laminate which contains a stretched foil and an unstretched foil. If a layer of plastic material is extruded directly against a stretched foil, the extruding operation must be carried out at a temperature which is too high so that the part of the laminate which includes the stretched foil no longer exhibits the advantages of a stretched foil.
The article by Breitbach et al. on pages 356-368 of "Kunststoffe" (Volume 61, 1971, H. 5) describes the advantages of laminates which consist of plastic foils. The authors discuss lamination with adhesive, so-called wax lamination and so-called hot lamination. The article does not propose to make laminates which consist of or contain stretched and unstretched plastic foils.
Published European patent application No. 0 122 495 of Koebisu et al. discloses a laminated film of an olefin polymer. The product of Koebisu et al. comprises a biaxially oriented polypropylene film and at least one olefin polymer film which is laminated to at least one surface of the biaxially oriented film. The inventors propose to bond the films with an adhesive or to resort to coextrusion.
Published European patent application No. 0 052 262 of Stegmeier et al. discloses a biaxially stretched polypropylene foil at least one side of which is coated with a layer of polyethylene. The application of the polyethylene layer or layers involves coextrusion, hot lamination or fusion coating.
German Auslegeschrift No. 1 294 005 of Kahn et al. proposes that lamination of foils be followed by stretching of the resulting laminate.
German Auslegeschrift No. 28 15 855 of Buzio et al. discloses a laminate consisting of a biaxially oriented polypropylene foil and a non-oriented polypropylene foil. The two foils are bonded to each other by a layer of adhesive.